A. From your comments, it appears that your new batching operation has eliminated a principal source of concrete strength variation—poor mixing. The best way to analyze your situation is to adopt a statistical evaluation of your testing program.
Your best guide is the American Concrete Institute (ACI) document, “Evaluation of Strength Test Results of Concrete,” developed by ACI Committee 214. This provides an introduction to evaluating concrete strength tests and includes statistical procedures users will find useful in interpreting test result variations.
This might be a good time to reacquaint your quality control team with the document. Committee 214 just completed a thorough review, updating several sections. When it was originally introduced in the late 1970s, most statistical calculations were conducted by hand. The revision now takes into account the effect of new computer programs that can quickly analyze data.
It's important to remember that your test results represent the potential strength of the concrete. To develop a statistically accurate trend analysis, it's important to adopt an unbiased testing program. This includes establishing a random testing procedure. For example, you shouldn't only sample each day's first load of a particular mix design.
At the ACI Convention in November, Committee 214 presented a technical session on the Evaluation of Concrete Test Results, which provided information on how producers could use the statistical analysis proposed in the ACI 214 information to determine if a mixing operation is in control.
Committee member John Luciano, manager of product support at BASF Admixtures, demonstrated a software program that automated and simplified the statistical calculations found in ACI 214. The program allows users to enter raw test data and then calculate important information such as the graph of Probability of Low Strength.
Luciano has posted this analysis program on his company's Web site for downloading. You can find it at www.basf-admixtures.com in the Support section.Floor Coating Problems
Q. We are about to submit a bid for concrete that will be the slab for a new high school gym floor. We've heard that there have been problems on these types of flooring projects because of concrete dryness and flooring adhesive issues. Where can we research the most current technical advice on these projects?
A. Costs associated with construction delays, pending legal action, and loss of productivity resulting from flooring failures on slab-on-grade concrete are the leading claim categories in the concrete industry. Several years ago, the American Society of Concrete Contractors (ASCC) invested in a research project to help support their members in promoting the best practices in this area. Under the leadership of Bruce Supprenaut, ASCC developed “Moisture-Sensitive Floor Coverings on Concrete Slabs Position Paper #10,” describing how contractors view the problem.
Fortunately, there's an even better resource. Last month, ASCC released the Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials. The book covers concrete moisture basics and testing, and vapor retarders/barriers, along with design and construction recommendations.
ACI 302 Committee and ACI's Technical Activities Committee reviewed the 42-page guide. You can buy the book at www.ascconline.org.